Less-invasive intravascular operation of inserting a catheter percutaneously into the blood vessel for treatment of vascular lesion is mainly practiced recently for reduction of physical and temporal loads applied on patients. Examples of such operation include a percutaneous blood vessel-forming operation of enlarging the stricture site of blood vessel or removing thrombus from thrombus-clotted blood vessel by applying negative pressure to the catheter, an intravascular operation called embolotherapy of injecting a thrombolic substance or a coil to lesions such as aneurysm, arteriovenous malfunction, and hemangioma often found in brain and other blood vessels.
The catheters for use in the percutaneous blood vessel-forming or intravascular operation demand, for example, the following properties.
(1) Favorable Operational Convenience and High Safety:
The catheter should be delivered rapidly and accurately to a lesion in the body without damage on vascular wall and others during insertion into the body. It is related to the efficiency of positional adjustment, for example, for reliable transmission of the surgeon's operation from the proximal region to the distal region during insertion or withdrawal of the catheter into or out of blood vessel and others.
Specifically, the positional adjustment efficiency is dependent on the resistance of catheter to elongation (stretch resistance), the efficiency of transmitting the torque from proximal region on the catheter, and the like.
(2) Preservation of Lumen Diameter and Reduction of Frictional Resistance of Lumen Surface:
These properties are related to the lumen lubricity and pressure resistance of catheter that are needed to inject a medicine such as contrast medium or thrombolic substance into the patient's internal lesion through the tube lumen, which is formed by a catheter internal layer resin, to remove intravascular substances such as thrombus and debris (foreign matter) smoothly out of the blood vessel for example by suction, or to deliver another treatment device such as guide wire less forcibly.
Specifically, it is needed to have a favorable guide wire-delivery efficiency of inserting and withdrawing the catheter smoothly along a previously-installed guide wire without damaging the internal wall of bent blood vessels. The catheter is desirably compatible with blood or organs. In addition, the catheter tube also demands favorable kink resistance prohibiting folding in the curved or bent area of blood-vessel when the distal end of catheter tube reaches a desired position and then the guide wire is removed and favorable distal-region flexibility keeping the shape favorable for the blood vessel without damaging it. It is necessary to reduce the external diameter of the tube for reduction of the frictional resistance with vascular wall and also to thin the internal layer of the catheter extremely (thinning) for prevention of decrease in medicinal flow rate and also in the flow resistance of intravascular substances and for increase of internal diameter.
To satisfy the requirements described above, this kind of catheter has, for example, a configuration having a multi-layer structure consisting of internal and external layers containing a resin and/or an elastomer and a reinforcement layer having a metal wire and/or a synthetic resin wire and also having a highly flexible distal region and a relatively rigid main body, wherein the rigidity of the catheter tube varies gradually from the distal region to the main body.
In particular, a fluoroethylene polymer (fluoroplastic), for example, is used as the material for the internal layer, for reduction of the frictional resistance on the lumen surface and thus for easier movement of the medicine or treatment device through the lumen. Specifically, tetrafluoroethylene polymers (PTFE's) superior in chemical resistance, low friction, and electric insulation efficiency are used widely.
As for the production method for catheter using PTFE, for example, Patent Documents 1, 2 and 3 disclose methods (dipping methods) of producing a catheter, by coating a core wire such as copper wire with a PTFE dispersion, forming an external resin layer thereon after sintering, and withdrawing the core wire.
However, the methods disclosed in the Patent Documents 1 to 3 gave catheters unsatisfactory in tensile strength because of absence of orientation and also in lumen lubricity during insertion or withdrawal of the guide wire because of separation of the PTFE particles. In addition, the catheters obtained by the method disclosed in the Patent Documents 1 to 3, which are not oriented, were unsatisfactory in kink resistance and positional adjustment efficiency (including stretch resistance and torque-transmitting efficiency). Further, it was necessary to repeat coating and sintering of a dispersion several times before obtaining a PTFE tube having a desired wall thickness, which caused problems of low productivity and local fluctuation in thickness by uneven coating.
In addition to the dipping methods disclosed in Patent Documents 1 to 3 above, extrusion molding methods are also used for producing catheters. The extrusion molding methods include melting extrusion molding and paste-extrusion molding. PTFE resins, which are very higher in melt viscosity, are molded not by melting extrusion molding, but normally by paste-extrusion molding.
For example, Patent Document 4 discloses a PTFE-based resin tube and a production method thereof, by extruding a PTFE-based resin, for example, by tube molding such as paste extrusion, sintering the extruded resin, and then, drawing and thus thinning the resin tube in the longitudinal direction by a drawing machine.
Patent Document 5 discloses an amorphous fluorine polymer superior in tensile strength allowing paste-extrusion molding, a PTFE composition containing a fluorine-containing chemical, and a production method thereof.    Patent Document 1: Japanese Unexamined Patent Publication No. 2000-316977    Patent Document 2: Japanese Unexamined Patent Publication No. 2000-51365    Patent Document 3: Japanese Unexamined Patent Publication No. 2002-45428    Patent Document 4: Japanese Unexamined Patent Publication No. 2004-340364    Patent Document 5: Japanese Unexamined Patent Publication No. 2000-136280